Before you go spending money on pumping your septic tank out.... Do you have a tank type water heater?

I hate to say that I dont know what type of water heater we have, it's a big round "tank" with pipes running upward from it.

And if this is a "tank" type water heater? what's your next thought/suggestion?

Yeah its a tank type. The sulfur smell that you are talking about, does it only happen when you run hot water for a shower?

If the answer is yes then the sacrifical anode riser in your water heater has gone out. There should be a hex nut about a inch or so in diameter on top of the heater.Anode Rod: A sacrificial rod installed in a water heater that protects the tank from corrosion, helping to extend the life of the tank. More than likely this is what is causing the smell.

Is the smell limited to just one bath? Or throughout the house? How close is the heater to that bathroom?

It could also be a badly clogged drain, too, giving off that smell but I'd check the anode too.

It sounds like the formation of hydrogen sulfide gas. The anode is one item to look at, but usually the reaction is biological. You've probably got thermophilic (heat loving) bacteria reducing sulfate to hydrogen sulfide. Flush tank until spigot is clear and raise the water temp to atleast 140F for at a minimum of 8 hrs.